1. Field of the Invention
The invention is generally related to fixed offshore platforms and more particularly to the installation of an offshore jacket.
2. General Background
In the offshore oil and gas industry, a variety of floating and fixed structures are used for drilling and production operations. Fixed offshore structures are generally comprised of a jacket and a platform. The jacket is an open space frame structure formed from tubular steel and extends from the sea floor to above the water line. The platform is generally formed from one or more modules that contain the living and work areas and support the derrick and associated drilling and production equipment. The platform is supported on top of the jacket and the combination of the jacket and platform is often referred to as the offshore platform.
A key design constraint for fixed offshore structures is that there be no substantial dynamic amplification of the platform's response to waves. This is accomplished by designing the platform to have natural vibrational periods which do not fall within that portion of the range of wave periods representing waves of significant energy. The several modes of platform vibration which are generally of greatest concern in platform design are pivoting of the structure about the base (commonly termed "sway"), flexure (bending) in the vertical plane, and torsion about the vertical axis. For deep water applications, greater than about four hundred meters, the conventional rigid structure design becomes uneconomical. It then becomes necessary to use compliant platforms that give a sway period greater than the range of periods of ocean waves containing significant energy. A compliant platform uses its own inertia and flexibility to increase the sway period, thereby reducing the dynamic amplification of the platform's response to waves, which in turn reduces the structural steel needed and higher cost associated with a given increase in water depth. This is accomplished by the use of flexpiles which are tubular steel members which are attached to the legs of the jacket by a combination of a top rigid connection and slip joints along the length of the flexpiles and jacket legs. The length of the flexpiles is dependent upon the of the required flexibility of the combined tower and platform.
Installation of fixed and compliant offshore structures may be accomplished in several different ways.
For a typical shallow water structure, the jacket is set in place on the sea floor and piles are driven through the legs a suitable depth into the sea floor. The piles are typically at an angle from the vertical and are connected to the top of the jacket. Once driven in place, the piles are grouted to the legs of the jacket.
For a typical deep water structure, the jacket is provided with sleeves that are attached to the lower level of the jacket such that the sleeves are at or just above the sea floor. Piles are driven through the sleeves into the sea floor through the sleeves. These piles are normally referred to as skirt piles. The skirt piles are grouted to the sleeves once driven to the desired depth. With this configuration, buoyancy can not be provided in the sleeves as all bulkheads must be removed prior to pile installation.
In another offshore platform disclosed in U.S. Pat. No. 4,669,918, foundation piles are driven into the sea floor before the jacket is lowered into position. The jacket is then lowered into position and the jacket legs are stabbed into the foundation piles. The jacket legs are provided with bulkheads at a selected distance from the bottom of the legs such that the bulkheads rest upon the foundation piles before the bottoms of the legs rest upon the sea floor. In this manner, the bulkheads in the jacket legs and foundation piles have metal-to-metal contact and the jacket and platform are supported directly upon the foundation piles immediately upon installation of the jacket.
There are certain disadvantages to the installation method of the patent described above. The possibility of damaging the foundation piles or the flex piles is increased during the installation of the jacket, especially in deep water depths where control of the jacket section during lowering and placement on the bottom is difficult. A disadvantage of placement directly onto skirt piles is that the piles must be leveled before the jacket is placed onto the piles.
Compliant towers typically require twelve or more foundation piles. Achieving adequate levelness across twelve or more foundation piles becomes very difficult and impractical in deep waters.